Gas phase fluidized bed polymerization processes find extensive use in the preparation of olefinic polymers, particularly polyethylene and polypropylene. The properties of the resultant polymer are often determined by the catalyst used. From time to time it may be desired to produce a different polymer in a given gas phase fluidized bed polymerization apparatus. In such event, the catalyst change is effected by what is known as transitioning. In transitioning, the feed of the first catalyst is ceased and replaced with the feed of a second catalyst to the polymerization zone. Since both catalysts will be present during the transitioning, the resultant product will thus not meet the product specifications for the polymer made with the first catalyst nor for the polymer made from the second catalyst. Over a period of time, the first catalyst is consumed and discharged from the reactor and polymer made from the second catalyst predominates. In order to meet the product specifications for polymer made with the second catalyst, normally at least three bed turnovers occur. A bed turnover occurs when the amount of product discharged from the reactor equals the amount of polymer contained in the fluidized bed. Considering that many of the world class gas phase fluidized bed polymerization processes contain 50 metric tones or more of polymer within the fluidized bed at any given time, the amount of off specification material produced can be quite substantial during a catalyst transition.
One way that has been contemplated for reducing the amount of polymer produced during a catalyst transition is to lower the height of the fluidized bed. Unfortunately, in gas phase fluidized bed polymerization processes, lowering bed height is not without problems.
The essence of the problem is to maintain the walls of the reactor and the expanded zone above the reactor relatively free from polymer deposits. The upward flow of the fluidizing gas entrains small particles which are carried into an expanded section of the reactor vessel above the fluidized bed. In the expanded section, the reduced gas velocity results in these smaller particles disengaging from the gas and returning to the bed. Some of these disengaged particles will contact and adhere to the walls of the expanded section or reactor. Since the particles in the bed contain active catalyst and the gas contains reactive monomer, the particles can continue to grow. The adhering particles may continue to polymerize forming aggregates, or sheets, of polymer. These aggregates of polymer, if allowed to form, may shed and fall into the fluidized bed. The agglomerates may not meet product quality specifications and, may be of such a size as to disrupt the fluidization of the bed. This disruption could become catastrophic and lead to a reactor shutdown.
The frequency and size of polymer deposits has been found to be attenuated by maintaining the top of the bed proximate to the expanded section such that the bed surface disruptions associated with the fluidization of the bed result in a substantial flow of larger particles into the expanded section. For instance, in commercial scale units which may have diameters as great as 5 meters and height exceeding 20 to 25 meters, the fluidization activity within the bed becomes quite violent. The violent nature of the fluidization can be readily appreciated from the fact that the upwardly flowing gases through the fluidized bed coalesce to form bubbles having diameters in excess of 2 meters by the time they burst through the top of the fluidized bed. The gases are often flowing at a velocity of at least about 0.5, and sometimes in excess of 2, meters per second.
This flow of larger particles tends to scrub deposits of polymer from the wall prior to forming deleterious agglomerates. Accordingly, dropping the level of the bed would reduce the amount of larger particles flowing into the expanded section and could significantly increase the risk of undesirable agglomerates being generated on the walls of the expanded section.
Reducing the velocity of the upwardly flowing gas can reduce the amount of smaller particles being carried above the fluidized bed. However, velocity reduction poses a problem in maintaining desirable fluidization of the bed. As the bed height is reduced, the pressure drop to the fluidizing gas is proportionately reduced since the pressure drop caused by the bed is directly related to the weight of the bed. But, the pressure drop to the gas as it passes through the grid changes substantially since pressure drop is a function of the square of the velocity of the gas. Thus, the ratio of the pressure drop across the distribution plate to total pressure drop changes with changes in gas velocity.
By way of background, the pressure drop across the grid, or distribution plate (.DELTA.P.sub.p), should be sufficient to assure a substantially even distribution of the fluidizing gas over the cross section of the fluidized bed. This distribution is necessary not only for the fluidization of the bed but also to prevent particles from agglomerating or other untoward consequences in the polymerization environment. Typically, .DELTA.P.sub.p should be greater than about 15 percent of the total pressure drop (.DELTA.P.sub.T) across the bed and distribution plate. An economic balance exists, if the size of the openings in the distribution plate are smaller in order to provide a desired .DELTA.P.sub.p /.DELTA.P.sub.T ratio over a range of fluidizing gas velocities, then greater the energy costs will be incurred to provide desired fluidization at advantageous fluidizing gas velocities at full bed height. Typically prior to this invention, the distribution plate is designed as a compromise between achieving energy savings and the range in operable fluidizing gas velocities and thus the range in bed heights.
Processes and apparatus are therefore needed to enable fluid bed operations over a wide range of bed heights to facilitate star-up and especially to reduce the amount of off-specification material produced during catalyst transitions.